The present invention relates to counterbalance loading device and more particularly to a compensated main piston drive device for applying small loads to a test sample which includes a counterbalance piston to compensate for the weight of the main drive piston so that the lower limit of the applied load is not constrained by the weight of the hardware used.
The present invention has found particular application in the field of measuring the absorbency of various absorbent materials of the kind that would be used in disposable diapers, bandages, sanitary napkins, incontinent pads and the like. During use there is often a compression load on such devices, for example a baby might be sitting on the diaper or a bandage might be wound with some tightness about the body. Therefore, when one is testing the absorbency of a particular material, it is useful to place a compression load on the material and to vary the load according to specific time sequences or other parameters. The absorbency data collected under these conditions will more accurately reflect actual use.
When one is actually testing the absorbency of such material in a laboratory, it is useful to be able to load the material with a prescribed load for a prescribed period of time and to record the data automatically. Especially in the laboratory environment, it is useful to be able to quickly and easily change the applied loads and the time sequences without having to waste valuable laboratory time in resetting the test apparatus.
In the past, such tests had been conducted by a laboratory technician who would place a test sample on a flat surface under a pressure plate and then manually place known weights on the pressure plate. The technician observes the sample and measures it thickness at regular time intervals. There is a correlation between the thickness and absorbency of the test sample as pointed out in the testing procedure that is employed by a Gravimetric Absorption Tester, described in U.S. patent application Ser. No. 149,214 filed on the same day as this application by Wesley J. McConnell, and assigned to the same assignee of this application. The said McConnell application is incorporated herein by reference.
If the test sample is small, the weight of the pressure plate may be unimportant. However, for larger samples, for example, three or four inches in diameter, the weight of the pressure plate may significantly effect the lower limit of the applied load. This further inhibits the use of automatic testing procedures.
It would be desirable to have an automatic loading device in which the weight of the pressure plate and the load-applying apparatus is automatically compensated so that the lower limit of applied load is not unreasonably restrained. It would also be desirable to have an automatic loading device which could be automatically programmed to apply a variety of desired loads for a variety of time periods.